The present invention relates to a process for preparing a structure having enhanced magnetoresistance on the basis of the system of materials Cu--Co. A process of this type follows from "Appl. Phys. Lett.", Vol. 58, No. 23, Jun. 10, 1991, pp. 2710 to 2712. The present invention further relates to the use of such a structure and to a magnetoresistive sensor prepared by the process.
The general structure and the mode of operation of magnetoresistive sensors having thin films made of ferromagnetic transition metals are explained in more detail, for example, in the publication "Sensors", Vol. 5, 1989, pp. 341 to 380. However, the largely magnetostriction-free layers disclosed there, which comprise, e.g., a special NiFe alloy "Permalloy") or a special NiCo alloy, only exhibit a relatively small magnetoresistive effect M.sub.r of approximately 2 to 3%. The variable M.sub.r in this context is in general defined as follows: M.sub.r =[R(O)-R(B)]/R(O), R(B) being the ohmic resistance of the structure in an external magnetic field having the induction B, and R(O) being the corresponding resistance in the absence of a magnetic field.
Any enhancement of this magnetoresistive effect would be thus be of interest in order to be able to produce sensors having an improved signal-to-noise ratio and to extend the field of application of these sensors. An enhanced magnetoresistive effect has been demonstrated in a number of multilayer systems such as Co/Cu, Co/Cr and Fe/Cr (e.g., see "Phys. Rev. Lett.", Vol. 64, No. 19, May 7, 1990, pp. 2304 to 2307). This is based on the fact that the non-magnetic interlayer gives rise to exchange coupling (exchange interaction) which depends on the thickness of the interlayer (cf. contribution B.C.07 to the "35th Annual Conference on Magnetism and Magnetic Materials" (MMM Conference), San Diego, USA, Oct. 29 to Nov. 1, 1990). The exchange coupling is responsible for the magnetic behavior (ferromagnetic-antiferromagnetic) of the multilayer system.
Accordingly, multilayer systems having different directions of polarization of the superimposed ferromagnetic individual layers, which are separated by non-magnetic layers, may produce an enhanced magnetoresistive effect. This effect, which for layered Cu--Co thin-film structures at room temperature may amount to up to M.sub.r =40% (cf. the literature reference, mentioned in the introduction, from "Appl. Phys. Lett." 58), is therefore also known as the "Giant Magnetoresistive Effect" ("Phys. Rev. Lett.", Vol. 61, No. 21, Nov. 21, 1988, pp. 2472 to 2475).
The limitation to multilayer systems and the fact that the effect is strongly dependent on the very low thickness of the magnetic and non-magnetic layers, respectively, in the nanometer range makes great demands on process technology, however, and restricts the field of application to thin-film structures. Moreover, suitable substrates are required as carriers for the multilayer system.
In addition, experiments have been carried out according to which a magnetoresistive effect can also occur in granular systems (cf. "Phys. Rev. Lett.", Vol. 68, No. 25, 1992, pp. 3745 to 3752). In this known process, CuCo alloy layers are produced by simultaneous sputtering of the elements, a subsequent heat treatment being employed to generate nanocrystalline (magnetic) Co precipitations in a (non-magnetic) Cu matrix. The magnetoresistive effect M.sub.r which can be measured in these thin films is, however, no greater than at most 7% at room temperature.